Speaker
Description
The KSTAR tokamak is now under transition period of the plasma facing materials from carbon to tungsten: the carbon tiles as the bottom divertors are replaced with tungsten mono-blocks before 2023. The remaining PFCs such as passive stabilizers, top divertors and limiters will be coated tungsten on the existing carbon tiles for the upcoming 2027 experimental campaign. Accordingly, the normal densities with moderate or zero gas fueling but neutral beams tend to increase. However, high densities with the considerable amount of fuel injections are not clearly increased compared than those of carbon wall era. The density signals used hereafter are basically line-averaged ones $\overline{n_e}$ obtained by tangential two-color interferometer (TCI). The maximum achievable density was up to 0.8 of Greenwald density ($n_{GW}$) as a record number and the $n_{GW}$ has never been achieved in the KSTAR tokamak both under the full carbon PFCs or the recent bottom W divertors. This is due to the mainly two reasons which are combined together : 1) the volume of the KSTAR plasmas (12 m$^3$) are very small compared to the size of the vacuum vessel (110 m$^{3}$). Whenever the H-mode discharge is changed back to L-mode even with strong gas puffing to push the density up to the Greenwald limit, the density drops abruptly down to below 0.5 $n_{GW}$ and the MARFE instability pops up causing immediate major disruptions, and 2) hence the density limit of the KSTAR is inevitably the same term to the H-mode density limit (HDL) which is critically influenced by the total heating power magnitude to sustain the H-mode over the power threshold. In this regards, there will be two ways to achieve the Greenwald density on KSTAR : 1) to reduce the vacuum vessel size to sustain the density as high as possible even after the H-L back-transition like other machines e.g. JET and ASDEX-Upgrade, 2) to enhance the total heating power to sustain H-mode until the density reaches $n_{GW}$.